Process of plating with chromium



Patented June 22, 1926.

UNITED STATES 'lmrlalxrr OFFICE.

JOHN nnnm'nosnowron, or JERSEY our, NEW JERSEY, ASSIGNOR ro onnommm rnonucrs CORPORATION, or WILMINGTON, DELAWARE, A CORPORATION or DEL- AWARE.

No Drawing.

This invention relates to process of chrome plating; and it comprises a method of producing hard, dense and continuous coatings of chromium by electr c-deposition wherein a chrome plating bath containing chromate of chromium receives an addition of a' salt of a metal more negative than chromium in the electrochemical series, such as iron, andbath conditions are so adjusted as to deposit chromium practically to the exclusion of the other metal; all as more fully hereinafter set forth and as claimed.

The properties of the'metal render it va1- uable as a surface coatingfor other softer and more corrodible metals; and various processes, and notably that of Grube, Patent No. 1,496,845, are in use for producing such a coatingby plating from an acqueous bath. In the Grube bath,'chromic acid (CrO is present and is used for replenishment as the bath loses chromium. From these baths, chromium coatings maybe electrically deposited having all the desirable properties. The best of the baths contain chromium in two states of oxidation, as CrO, and as hydrat-ed Cr O and the two oxids may be combined as chromate of chromium.

While these processes are good and produce good coatings they require skill in use and are open to improvement in throwing power; which for the present purposes is the property producing even coatings irrespective of surface configuration in the cathode. With any electroplating'bath there ,is always a tendency for a deposit to occur preferentially on projecting portions of an article forming a cathode as compared with hollows, crevices and sheltered portions. The better the throwing power of a bath, the more this tendency is restrained.

In the course of an extensive series of experiments on improving the throwing power of these baths by various addition agents, organic and inorganic, the unexpected discovery was made that throwlng power as regards chromium was considerably improved by the presencein the bath of various other metals more negative than chrom ium in the electrochemical series; that is, by the presence of metals which normally are more readily plated out of solution than is chromium itself. In a. bath containing (11-0 and chromate of chromium however.

PROCESS OF PLATIN (:1- WITH CHROMIUM.

Application filed September 10, 1925. Serial No. 55,622.

and of the right pH concentration, theseother metals do not deposit and on the other hand chromium is thrown down in coatings of desirable properties both from a physical and chemical point of view. Deposits may be had ranging in color from bright highly lustrous metal through a frosted condition and down to a dull gray. Unexpectedly the voltage required is lower than that heretofore considered necessary in the deposition of chromium from these chromic acid baths and the cathode current efiiciency is higher; The deposits obtained are substantially pure chromium in spite of the presence of the other metal which is normally more easily reducible.

It has been found'th'at the phenomenon in question exists with all the salts of the heavy metals more negative than chromium in the electrochemical series when present in certain definite concentrations. The salts may be added to the bath as sulfates, chlorides, bromides, carbonates, etc., and the amount present may vary from less than a gram per liter up' to about 30 grams per liter, that is, about 3 per cent.

- In thepresent invention I employ for the deposition of chromium an electrolyte containing chromium chromat-e and salts of an,

other metal in proportions within the stated range.- The pH concentration of the bath should be such as to insure an excess of chromium hydroxid over the amount necessary' to neutralize any free acid other than CrO present in the electrolyte, while the concentration of CrO should be at least twice that of the hydrated Or o and of the metallic salt-used. Under these conditions.

the bath contains chromium chromate and there is a sufficient concentration of chromium ions to permit free deposition of chromium. Insoluble anodes such as lead or oxids thereof may be used or-there may be boiling and then stirring into the hot soluticn 12 grams of commercial hydrated chromic oxid, C1'(OH) When the latter 1s completely in solution 6 grams ofcommercial hydrated ferrous sulfate,

FeSO .7H O,

temperature for electrolysis of this solution is between 35 and 45 C. A suitable voltage is from 2.5 to 5 volts; the particular voltage depending on' the size of the cathode, its shape and. the thickness of deposit desired. Ordinarily it is desirable to have a current density of 10.05 to 0.2 amperes per square centimeter. During electrolysis solid CrO is added from time to time so as to keep the chromium content of the electrolyte substantially unchanged. Since the iron is not thrown down no replenishment .of this is necessary except as regards mechanical losses.-

In practice the bath is analyzed from time more ferrous sulfate is added. Instead of I usin ferrous sulfate as in the above exampIe other metallic salts may be employed.

lVith' nickel sulfate similar results are obtained. Chromium plating from a. bath containing nickel sulfate to the extent of 6 grams per liter (0.6%) with 325 grams of CrO and 12 grams of Cr(OH),. on analysis shows no trace of nickel. Like results are obtained Where the bath receives an addition of sulfates, chlorides, etc., of cadmium, tin, lead. antimony, bismuth, copper and mercury.

\Vhat- I claim is 1. In the production of chromium plating baths of better throwing power, the process which comprises adding a small proportion of a soluble salt of a metal more negative than chromiumin the electrochemical series to a chromium plating bath containing chromium in two stages of oxidation.

A process such as described in claim 1,

where the salt added is a salt of iron.

3. A process such asdescribed in claim 1, wherein the amount of metal salt present together with the amount of chromium in the lower state of oxidation is not more than one-half the concentration of U10 In testimony whereof I aifix my signature.

JOHN MERLE HosDoWioH. 

